Comminution of coal, ores and industrial minerals and rocks

ABSTRACT

Crushed particles of coal, ores or industrial minerals or rocks are comminuted by feeding them through a feeder (14) into a cyclic stream (19, 22, 38, 39, 41) of cryogenic process fluid such as liquid carbon dioxide and conducting the process stream with the entrained mineral particles to a comminuter (17) and through a zone therein of mechanically generated high frequency vibratory energy, preferably ultrasonic. The comminuter (17) may be multistage with means for re-cycling oversize mineral particles and, after leaving the comminuter (17) the process stream (38) is conveyed to a separator (18) for extracting the comminuted particles and re-cycling the cryogenic fluid to the feeder (14). The low temperature of the process stream is maintained by refrigerating means (16) and losses of the fluid are made up by supplementary fluid fed to the stream.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a method of and apparatus for the finecomminution of coal and other mineral matter such as ores of basemetals, iron ore and, more generally, all materials described asindustrial minerals and rocks (hereinafter referred to as "minerals").

(2) Prior Art

A process and apparatus for the ultrasonic comminution of solidmaterials are described in the specification of U.S. Pat. No. 4,156,593of W. B. Tarpley Jr., and a process of ultrasonic homogenisation oremulsification is disclosed in the specification of U.S. Pat. No.4,302,112 of P. R. Steenstrup. A process and apparatus for continuationby sonic high frequency impacting or crushing are described in thespecification of Australian Pat. No. 544,699 of A. G. Bodine.

SUMMARY OF THE PRESENT INVENTION

The present invention has for its objects the provision of a method andapparatus by means of which the fine comminution of minerals may becarried out particularly efficiently. According to the invention amineral, such as coal for example, which has been crushed in ahammermill or like apparatus, is introduced by a feeder to a cyclicstream of cryogenic fluid, such as liquid carbon dioxide or liquidnitrogen for example, by which the entrained mineral particles arecarried through a comminutor applying mechanically generated highfrequency vibratory energy, the cryogenic fluid and comminuted mineralbeing then conducted to a separator by which the comminuted mineral isseparated from the fluid and discharged, the fluid being re-cycled tothe feeder. In a primary heat exchanger the fluid from the feeder ispre-cooled by fluid passing from the comminuter to the separator, andthe fluid is further cooled to the required operating temperature beforereaching the comminutor by refrigerant in a secondary heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a diagrammatic illustration of a continuous comminutioninstallation according to the invention, and

FIG. 2 is a diagram of the comminuting apparatus of the installation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The installation shown in the drawings is devised for the comminution ofcoal, but it is to be understood that it is applicable, withmodifications if necessary or desirable, to the treatment of otherminerals as set out above.

The installation includes a primary crusher 10, which may be ahammermill or other known device capable of economically reducing coalintroduced to it to a size of the order of one to ten millimeters.

The crushed coal is conveyed by way of stream 11 to a storage hopper 12from which it is drawn as required and conveyed at ambient temperature,by way of stream 13, to a feeder 14.

The continuous comminution process involves the introduction of thecrushed coal to a cryogenic process fluid and its conveyance by thisfluid in sequence from the feeder 14, through a primary heat exchanger15, through a secondary heat exchanger 16, through a high frequencycomminuter 17, back through the primary heat exchanger 15 and to amineral-fluid separator 18 where the comminuted coal is discharged andthe cryogenic process fluid is recycled through the feeder 14.

Any of a number of cryogenic fluids may be used as the process fluid,liquid carbon dioxide being a suitable medium, as also is liquidnitrogen, although other elements or compounds that remain liquid belowabout -40° C. such as the inert gases or low molecular weight alkanes(methane to nonane for example) or mixtures of these, or, moregenerally, components of natural gas, may be used.

The continuous processing system has an internal operating pressureselected to suit the properties of the process fluid used; for exampleif carbon dioxide is employed, the internal operating pressure must bein excess of 5.11 atmospheres to maintain the carbon dioxide in theliquid state.

The feeder 14 may be a lockhopper or equivalent device capable ofintroducing the crushed coal received from the storage hopper 12 intothe stream of cryogenic process fluid which has been separated from thecomminuted coal in the mineral-fluid separator 18. The stream of processfluid and crushed coal carried thereby travels by stream 19 through theprimary heat exchanger 15 where it is pre-cooled as before described,and to the secondary heat exchanger 16 where it is further chilled, by asuitable refrigerant stream 20, 21, to the operating temperature of thecomminutor. The process fluid and entrained crushed coal are fed to thecomminutor 17 via stream 22, and supplementary cryogenic fluid is addedto the system, prior to the comminution process, by stream 23 to make upany losses of the fluid that may have occurred as a result of the finalseparation of the product from the process fluid, or as a result of anylosses of the fluid at any other point in the system.

Referring now to FIG. 2, the comminutor assembly 17 diagrammaticallyillustrated is of two-stage type. It is a sealed refrigerated unit, toprevent or reduce thermal losses in the system, and it includes a firstsump 24 into which is introduced the process stream 22 with entrainedcoal particles and also the supplementary process fluid via stream 23.From the sump 24 the slurry of process fluid and crushed coal isdirected by a pump 25 to a first ultrasonic comminution apparatus 26which may be of the type described in the specification of said U.S.Pat. No. 4,156,593 of W. B. Tarpley, Jr. The slurry of process fluid andcomminuted coal is then directed via stream 27 to a classifier 28 whichseparates from the slurry such coal particles which are of greater thanrequired size and which are returned by way of stream 29 to the firstsump 24 for re-treatment, the balance of the coal particles beingconveyed by process fluid in a stream 30 to the second stage of thecomminutor, being fed into a second sump 31, to which supplementaryprocess fluid is conveyed by stream 32 from stream 23. The slurry ispumped by a second pump 33 to a second ultrasonic comminution apparatus34, similar to the first such apparatus 26 and thence, by stream 35 to asecond classifier 36, oversize particles of coal being recycled bystream 37, to the second sump 31. A slurry of process fluid carryingfinally treated particles is directed via stream 38 through the primaryheat exchanger 15, as shown in FIG. 1, to pre-chill the downstreamprocess fluid of stream 19, the two streams being, of course, separatedin the heat exchanger. Finally the process fluid and comminuted coalparticles travels by way of stream 39 to the mineral-fluid separator 18,the separated comminuted particles exiting therefrom in stream 40, thecryogenic process fluid being re-cycled, via stream 41, to the feeder14.

As the process fluid may be contaminated by ingress of air at the feeder14, and by hydrocarbon gases adsorbed to or absorbed in the coalparticles, it is preferred that there be included in the cycle apurifier 42 for the elimination of these extraneous gases. A condensor43 may be introduced in the stream 41 from the mineral-fluid separator18 to the feeder 14.

It will be found that the effectiveness of the process of comminution ofthe mineral in the process fluid in zones of mechanically induced highfrequency energy density is very materially increased by the lowtemperature conditions at which the operation takes place. Suchconditions cause the development of internal thermal stresses andoverall embrittlement of the mineral particles to yield a continuousprocess for the comminution. The process is efficient in either or bothof the following respects:

(i) a reduction in the energy density required to achieve a particulardegree of comminution of unit mass of the mineral,

(ii) an increase in the degree of liberation of mineral substanceconstituents, one from another, that is achieved at a particular energydensity per unit mass of material. The enhancement of liberationsimplifies and reduces the cost of subsequent mineral separationprocesses.

The use, as a process fluid, of liquified relatively chemically inertgases such as carbon dioxide or nitrogen gives the comminution processthe advantage of preventing the oxidation of the mineral surfaces thatmay occur in conventional processes. This lack of oxidation will, incases such as coal agglomeration or sulfide flotation processes, makethe valuable minerals or components more readily separated from theremaining non-valuable components of the mineral mixture.

The use of hydrocarbon gases as the process fluid or the use of amixture of condensed hydrocarbon gases and liquid carbon dioxide will,in some mineral beneficiation processes, cause such alteration of thephysiochemical properties of the mineral surfaces as will rendersubsequent beneficiation or mineral separation processes more efficient.

Where the process fluid used is a suitable medium for further processingor beneficiation of the comminuted mineral mixture, the separator 18 maybe omitted and the slurry of the comminuted particles in the fluid maypass to a downstream process. In this case, of course, the cryogenicprocess fluid is fed to the feeder 14 from a source of supply ratherthan recycled from the separator 18 as before described.

I claim:
 1. A method of comminuting minerals in a continuous comminutionsystem including the steps of:(a) crushing the minerals to form mineralparticles; (b) conveying the mineral particles to a feeder; (c)separately conveying to the feeder a stream of cryogenic process fluidin the form of a liquified relatively chemically inert gas selected fromthe group consisting of liquid carbon dioxide and liquid nitrogen; (d)combining the mineral particles and said cryogenic process fluid andconveying the particles in the stream of cryogenic fluid to acomminutor; (e) passing said mineral particles and said process fluidthrough a zone in said comminutor comprised of mechanically induced highfrequency vibratory energy thereby comminuting said mineral particles;and (f) separating the comminuted particles from the cryogenic stream ofprocess fluid.
 2. A method according to claim 1 wherein the cryogenicprocess fluid, after separation of the comminuted particles therefrom,is recycled through the feeder, and feeding supplementary cryogenicfluid into the process stream to make up losses of fluid therefrom.
 3. Amethod according to claim 1 wherein the stream of cryogenic fluid,upstream from the comminutor, is pre-cooled in a primary heat exchangerby the cryogenic stream downstream from the comminutor, and thepre-cooled cryogenic stream is further cooled by a refrigerant in asecondary heat exchanger upstream of the comminutor.
 4. A methodaccording to claim 1 wherein the stream of cryogenic process fluid ispassed through a purifier for extracting from the stream air or gasesadsorbed to or absorbed in the mineral particles.
 5. A method accordingto claim 1 wherein the high frequency energy of the zone within thecomminutor is ultrasonic.
 6. A method according to claim 1 wherein saidcomminuted particles after leaving said zone are conveyed with saidstream of process fluid to a second zone of mechanically enhanced highfrequency vibratory energy for still further comminuting said particles.7. A method according to claim 1 further including the step ofmaintaining an internal operating pressure in the system at leastslightly greater than the pressure required to keep said process fluidin a liquified state.
 8. A method of comminuting minerals in acontinuous comminution system including the steps of:(a) crushing theminerals to form mineral particles; (b) conveying the mineral particlesto a feeder; (c) separately conveying to the feeder a stream ofcryogenic process fluid selected from the group consisting ofhydrocarbon gases, and a mixture of condensed hydrocarbon gases andliquid carbon dioxide; (d) combining the mineral particles and saidcryogenic process fluid and conveying the particles in the stream ofcryogenic fluid to a comminutor; (e) passing said mineral particles andsaid process fluid through a zone in said comminutor comprised ofmechanically induced high frequency vibratory energy thereby comminutingsaid mineral particles; and (f) separating the comminuted particles fromthe cryogenic stream of process fluid.